1. Technical Field
The present invention generally relates to an electric ground working vehicle including left and right wheels independently driven by left and right electric motors respectively, caster wheels being steering control wheels capable of being freely steered, and a working apparatus driven for ground working.
2. Related Art
Hitherto, there has been known a ground working vehicle equipped with a working apparatus driven for ground working such as lawn mowing and cultivation. In such a ground working vehicle, there has been studied an electric ground working vehicle including left and right wheels being main driving wheels independently driven by electric motors respectively, and caster wheels being steering control wheels capable of being freely steered.
For example, as a working vehicle, there is a self-traveling lawnmower vehicle which is provided with a lawnmower being a working apparatus, and which a worker boards to drive and operate the lawnmower. This is referred to as a riding lawnmower vehicle. As a lawnmower, there is known a lawn mowing rotating tool, for example.
The riding lawnmower vehicle is a kind of vehicle, but it is used for traveling not on a road, but on so-called off-road surfaces such as a garden. The riding lawnmower vehicle travels on the ground for the purpose of lawn mowing and includes a power source for driving wheels and a lawnmower. Among riding lawnmower vehicles, one which uses an electric motor as a power source for driving wheels is called a riding electric lawnmower vehicle. The electric motor is supplied with electric power by an electric power supplying source such as a battery. There is being studied a hybrid riding lawnmower vehicle in which a generator producing electricity using an internal combustion engine is mounted, and generated electric power is supplied to a battery.
Japanese Publication of International Patent Application No. 2006-507789, for example, discloses a hybrid power plant fitted with an integral engine and generator unit in which the engine shaft of an internal combustion engine is coupled to a rotor. The application also describes that in the lawnmower exemplified as a power plant, independent electric motors are coupled to a plurality of driving wheels to enable respective driving wheels to be independently controlled at variable speed, thereby allowing smooth starting, stopping, changing speed, changing direction, and turning of the lawnmower. As an example of turning by changing the independent speed of the driving wheels, there are cited examples in the above where electric motors are coupled to left and right rear wheels respectively.
U.S. Pat. No. 7,017,327B2 discloses a lawnmower having a configuration in which a deck motor for driving a lawn mowing blade, a left and right wheel motor for driving the independently controlled left and the right rear wheels, and a steering motor for steering the left and the right front wheels within a range of approximately 180 degrees around wheel axes by an electric power of an alternator connected to an engine, are arranged in the front of the lawnmower. In order to turn the lawnmower, a difference in speed between the left and the right rear wheels is calculated from the input of a steering control unit to control the wheel motor, and a steering signal is applied to the steering motor to control the position of the left and the right front wheels. Thereby, according to U.S. Pat. No. 7,017,327B2, the lawnmower can be turned without the left and the right rear wheels being steered.
As prior arts related to the present invention, there are known Japanese Patent Laid-Open Publication Nos. Hei5-122805 and Hei10-164708, as well as Japanese Publication of International Patent Application No. 2006-507789 and U.S. Pat. No. 7,0173,27B2.
As a method of turning a riding electric lawnmower vehicle, Japanese Publication of International Patent Application No. 2006-507789 discloses that the rotating speed of the left rear wheel is made different from that of the right rear wheel by electric motors independently provided on the left and the right rear wheels respectively. In U.S. Pat. No. 7,0173,27B2, steering is performed by causing the left and the right wheel motor to provide difference in speed to the left and the right rear wheel and the steering motor to control the position of the left and the right front wheel.
As a conventional riding electric lawnmower vehicle, there has been studied a riding electric lawnmower vehicle which includes left and right wheels being main driving wheels independently driven by electric motors, and caster wheels being steering control wheels capable of being freely steered, and the vehicle is instructed to turn using a steering wheel being a turn operating element, and to accelerate using a acceleration pedal being an acceleration operating element. In such a riding electric lawnmower vehicle, when a driver instructs the vehicle to turn through a turn operating element, a controller receiving a signal corresponding to the instruction causes electric motors corresponding to the left and the right wheels to generate a difference in rotating speed, rotating the left and the right wheels at different speeds, thereby turning the vehicle.
For such a riding electric lawnmower vehicle, a turning direction is instructed by operating a steering wheel, and a speed is instructed by operating an acceleration pedal. For this reason, when the vehicle is freewheeling or the vehicle is braked, that is, the acceleration pedal is not operated with a foot kept off the pedal when traveling, the electric motors are also stopped. Therefore, even if the driver instructs the vehicle to turn through the steering wheel while the vehicle is freewheeling, the riding electric lawnmower vehicle cannot cause the left and the right wheels to generate a difference in rotating speed, so that it is difficult for the vehicle to turn in the direction which the driver wants.
On the other hand, when the riding electric lawnmower vehicle is traveling on a slope with the vehicle rolling, that is, while the vehicle is rolling and tilts to one side, to the left or the right, with an axis which extends in the front to rear direction of the vehicle and passes the center of gravity, gravity acts so that the vehicle slants to one side, namely to the left or the right thereof, so that even if the driver instructs the vehicle to move in a straight line using the steering wheel when freewheeling, the vehicle turns downward, which makes it difficult to cause the vehicle to travel in the direction which the driver wants. Needless to say, even when the driver instructs the vehicle to turn using the steering wheel, it is difficult to turn the vehicle in the direction which the driver wants. Thus, improvement is required in the difficulty in turning the vehicle in the desired direction when freewheeling from the viewpoint of safe traveling of the vehicle. In addition, there needs to be improvement to the disadvantage that the vehicle continues traveling in the undesired direction without stopping when freewheeling, from the viewpoint of safe traveling of the vehicle. Means of solving such disadvantages are not disclosed in any of Japanese Publication of International Patent Application No. 2006-507789, U.S. Pat. No. 7,0173,27B2 and Japanese Patent Laid-Open Publication Nos. Hei 5-122805 and Hei 10-164708. Furthermore, such disadvantages may occur not only in the riding electric lawnmower vehicle described above, but also in an electric ground working vehicle with other working apparatus.